1. Field of the Invention
The present invention relates to polysulfone-based hollow fiber membranes and processes for manufacturing the same, more particularly, to polysulfone-based hollow fiber membranes comprising skin layers on their inner surfaces predominantly containing a vinylpyrrolidone-based polymer, which are adapted for blood treatment, and processes for manufacturing the same.
2. Description of the Prior Art
Recently separation techniques utilizing separatory membranes having selective permeabilities, such as ultrafiltration, reverse osmosis, gas separation or the like, have been brought into practical use and separatory membranes made of a material adapted for diversified applications have been placed in the market.
As materials for the selectively permeable membranes, use have been made of polymers such as cellulose-based, cellulose acetate-based, polyamide-based, polyacrylonitrile-based, polyvinyl alcohol-based, polymethyl methacrylate-based, polysulfone-based, polyolefin-based or the like. Since the polysulfone-based polymers, inter alia, have excellent physical and chemical properties, such as resistance to heat, resistance to acid, resistance to alkali, resistance to oxidation or the like, they have recently attracted attention as materials for separatory membranes in medical or industrial application.
However, since the polysulfone-based polymers are hydrophobic, selectively permeable separatory membranes made therewith have a poor water-wettability and a low performance in dry, as compared with selectively permeable separatory membranes made with a hydrophilic polymer.
Therefore, attempts have been made to improve the water-wettability of the selectively permeable separatory membranes made with a polysulfone-based polymer, by imparting hydrophilicity thereto. As one of these attempts, there have been proposed selectively permeable separatory membranes comprising a hydrophobic polymer such as polysulfone-based polymers or the like and a hydrophilic polymer such as polyvinyl pyrrolidones or the like and manufacturing processes thereof.
For example, Japanese Patent Application Publication No. 018,695/1990 discloses polysulfone-based separatory membranes containing a polyvinyl pyrrolidone having a molecular weight of at least 100,000 in an amount of 5-70% by weight based on the membrane and having a water-absorbability of at least 11%, which are manufactured by spinning a dope comprising a polysulfone, a polyvinyl pyrrolidone having a molecular weight of at least 100,000 and a solvent common to these polymers.
Japanese Patent Application Laid-open No. 61-093,801 discloses hollow fiber membranes adapted for blood treatment containing a hydrophilic polymer in an amount of 1-10% by weight and having a water-absorbability of 3-10%, which are manufactured by spinning a dope of a low viscosity comprising a hydrophobic polymer, a hydrophilic polymer and a solvent common to these polymers.
Japanese Patent Applications Laid-open Nos. 61-238,306 and 63-097,666 disclose processes for manufacturing polysulfone-based separatory membranes with a dope system comprising a polysulfone-based polymer, a hydrophilic polymer and an additive which is a non-solvent or swelling agent for polysulfone-based polymers.
Alternatively, Japanese Patent Applications Laid-open Nos. 63-097,205 and 63-097,634 disclose processes wherein the polysulfone-based separatory membranes manufactured according to the above-described processes are subjected to irradiation and/or heat treatment to make the hydrophilic polymer insoluble in water.
Further, Japanese Patent Application Laid-open No. 63-099,325 discloses polysulfone-based hollow fiber membranes having their inner surfaces smoothed by injecting an inside coagulating liquid containing at least 5% by weight of a water-solublepolymer when the foregoing dope system is spun from annular orifices.
Japanese Patent Applications Laid-open Nos. 61-238,834 and 63-099,325 disclose polysulfone-based porous membranes comprising 3-30% by weight of a hydrophilic polymer, having micropores of an average pore diameter of at least 500 angstroms and exhibiting a water-permeability of at least 1,000 ml/m.sup.2 .multidot.hr.multidot.mmHg, which are obtained according to the above-described manufacturing processes.
Japanese Patent Applications Laid-open Nos. 61-000,402 and 62-038,205 disclose separatory membranes predominantly comprising a hydrophobic polymer, asymmetrically structured to have a dense skin layer on one surface side comprising a mixture of the hydrophobic polymer and a hydrophilic polymer.
The above polysulfone-based separatory membranes comprising a hydrophilic polymer are provided with excellent properties, such as water-wettability, an improved water-permeability, capability of preventing the membranes from staining due to their adsorption of proteins, or the like. However, since their biocompatibility has not been studied thoroughly, these membranes are not always satisfactory in respect of an antithrombogenetic ability.
For example, Japanese Patent Application Laid-open No. 61-093,801 discloses polysulfone-based hollow fiber membranes suited for blood treatment. In this application, there is described that these hollow fiber membranes can restrain a complement activation on a low level. However, the capability of restraining a complement activation onto a low level is developable only in the case where the hollow fiber membranes are hydrophobic. Namely, it has been suggested that in the hollow fiber membranes disclosed in Japanese Patent Application Laid-open No. 61-093,801, their inner surfaces contacting with blood are not sufficiently made hydrophilic and still remain as hydrophobic. The hydrophobicity remaining on the surfaces contacting with blood causes platelets to readily deposit on the surfaces. If the platelets once deposit on inner surfaces of hollow fiber membranes, a substance to activate a blood coagulation system is released by aggregation and destruction of the platelets, whereby blood coagulation is brought about.
Alternatively, the polysulfone-based separatory membranes disclosed in Japanese Patent Application Publication No. 018,695/1990, since they have a water-absorbability of at least 11%, may absorb moisture in air during storing after fabrication. Therefore, a special equipment is required for preservation of these separatory membranes. Additionally, a high water-absorbability is attributed to a large content of a polyvinyl pyrrolidone in the membrane, so that it causes the mechanical properties and the water-permeability of the separatory membrane to deteriorate.
Further, as to the manufacturing processes of polysulfone-based separatory membranes disclosed in Japanese Patent Applications Laid-open Nos. 61-238,306 and 63-099,325 or the polysulfone-based separatory membranes disclosed in the Japanese Patent Application Laid-open No. 61-238,834, since the dope for forming membranes is incorporated with a hydrophilic polymer, the inner surfaces of the hollow fiber separatory membranes cannot be made sufficiently hydrophilic, similarly to the polysulfone-based hollow fiber membranes disclosed in Japanese Patent Application Laid-open No. 61-093,801. Additionally, those processes are suited for manufacturing separatory membranes having pores as large as at least 500 angstroms but not appropriate for manufacturing separatory membranes used for hemodialysis. In the manufacturing processes of polysulfone-based separatory membranes disclosed in Japanese Patent Application Laid-open No. 63-099,325, the inside coagulating liquid contains a water-soluble polymer which, however, only gives smoothness to inner surfaces of hollow fiber membranes and does not remain on the inner surfaces.
The polysulfone-based separatory membranes having an asymmetric structure disclosed in Japanese Patent Applications Laid-open Nos. 61-000,402 and 62-038,205 have only their dense skin layer made hydrophilic, so that the water-permeability of the separatory membranes in dry is extremely deteriorated. Besides, hydrophobic portions of the membranes adsorb proteins.
An object of the present invention, therefore, is to solve the above problems presented by the hitherto proposed polysulfone-based separatory membranes and to provide polysulfone-based hollow fiber membranes having an excellent biocompatibility with a water-permeability not deteriorated after drying, which are particularly suited for blood treatment.
Another object of the present invention is to provide processes for manufacturing the above polysulfone-based hollow fiber membranes.